Despite aggressive management, septic shock arising from gram-negative sepsis continues to be a leading cause of death in both surgical and medical patients. Death in such patients usually results from cardiovascular collapse and/or multiple organ system failure. One of the main components of gram-negative bacteria thought to play an integral role in causing septic shock is an outer wall constituent, endotoxin.
Endotoxins are high molecular weight complexes, associated with the outer membrane of gram-negative bacteria, that produce pyrogenic reactions upon intravenous administration. Endotoxin is shed from living bacteria and is also released into the environment when bacteria die and decompose. Since gram-negative bacteria are found in great numbers in air, water, and soil, bacterial endotoxin commonly contaminates raw materials and processing equipment used in the manufacturing of, for example, pharmaceuticals.
Bacterial endotoxin is a complex consisting of lipid, carbohydrate and protein. It is characterized by an overall negative charge, heat stability and high molecular weight. Highly purified endotoxin does not contain protein, and is a lipopolysaccharide (LPS). Depyrogenation can generally be achieved by inactivating or removing endotoxin, depending upon the physicochemical nature of the LPS. LPS consists of three distinct chemical regions, lipid A, which is the innermost region, an intermediate core polysaccharide, and an outermost 0-specific polysaccharide side chain which is responsible for an endotoxin's particular immunospecificity.
Bacterial endotoxins are known to have profound biological effects in animals and humans, and to cause severe medical problems when present. Symptoms include induction of high fever, activation of complement, and hypotension. It is critical to avoid endotoxin contamination in any pharmaceutical product or medical device which comes into contact with body fluids. High endotoxin levels in sera due to bacterial diseases, such as septicemia, are not easily treated. Antibiotic treatment of the infection only kills the bacteria, leaving the endotoxin from their cell walls free to cause fever.
The horseshoe crab Limulus polyphemus is particularly sensitive to endotoxin. The cells from their hemolymph (amebocytes) undergo a complex series of biochemical reactions resulting in clot formation, analogous to mammalian blood coagulation. This phenomenon has been exploited in the form of bioassays sensitive to very low endotoxin levels. Currently, a bioassay of this type is the method of choice for monitoring pharmaceutical manufacturing and is termed Limulus Amebocyte Lysate (LAL). See U.S. Pat. Nos. 4,276,050, 4,273,557, 4,221,866, 4,201,865, 4,038,147, 3,944,391 and 3,915,805, each of which is incorporated herein by reference.
It has long been observed that once endotoxin interacts with LAL the toxin is not recoverable from the clot. See Nachum et al, Journal of Invertebrate Pathology, 32:51-58 (1978). This observation led investigators to postulate two alternative explanations. Either the endotoxin is enzymatically degraded during clot formation or it is bound by some factor causing it to lose toxicity. The present inventors initiated experiments to extract the endotoxin inactivating factor from the LAL.
Other research groups have experimented with endotoxin binding proteins, also referred to as anti-LPS factor. To the inventor's knowledge, the following publications resulting from work in this area are the most relevant to this invention:
Tanaka et al, Biochem. Biophys. Res. Comm. 105, 717-723 (1982),
Iwanaga et al, International symposium on Pyrogen, 84--84 (Jun. 23-26, 1987),
Aketagawa et al, J. Biol. Chem. 261, 7354-7365 (1986),
Hao, U.S. Pat. No. 4,677,194 (Jun. 30, 1987), and
Nachum et al, J. Inv. Path. 32, 51-58 (11978).
Tanaka et al, Iwanaga et al, and Aketagawa et al each conducted research on an anti-LPS factor or endotoxin binding protein isolated from a horseshoe crab system. Based on experimental work done in the inventors' laboratory, it appears that a protein involved in the present invention is the same as that isolated by Iwanaga et al and Tanaka et al. However, these publications do not say anything about pharmaceutical utility of the endotoxin binding/neutralizing protein, and it is difficult to predict in vivo activity based on in vitro experimentation. In fact, neither of these references suggests a practical utility for the anti-LPS factor, and in view of the unpredictable nature of in vivo activity, it has previously not been appreciated that the endotoxin binding/neutralizing protein could be used in a pharmaceutical composition. Furthermore, none of the references disclose the use of the endotoxin binding/neutralizing protein for an endotoxin assay, as disclosed in the present invention. It is notable that the present inventors have also discovered certain endotoxin binding/neutralizing protein variants which have amino acid structures that are different from the anti-LPS factor disclosed in the above-described publications.